The Standard Penetration Test (SPT) is an in-situ testing technique that drives a sampler into the ground at the bottom end of a drill hole (or borehole) during subsurface exploration. The test can yield a measure of the soil resistance to the penetration of the sampler under the impact of a free drop hammer from a constant height.
There are two operators to conduct the test operations. As shown in FIGS. 1 and 2, the primary operator uses the power of the drilling rig and the steel wireline above the derrick to lift or drop the hoist hook. The secondary operator couples or decouples the hoist hook either with the top of a drill rod (FIG. 1) or with the steel chain of a impact hammer apparatus (FIG. 2). The impact hammer apparatus includes the steel chain, a X-clamp, the hammer and the guide rod. The guide rod has a lower anvil at its bottom, an upper anvil at its top, and a steel chain. The hammer has a cap for clamping by the X-clamp. The testing at a drill hole depth follows the following three processes in a real time sequence.
At first, the sampler coupled to a drill rod in series has to be inserted into the drill hole (FIG. 1). The sampler has to reach the bottom of the drill hole. If the length of the drill rod whose bottom end is coupled with the sampler cannot make the sampler tip to reach the bottom of the drill hole, a second drill rod will be added to the top of the first drill rod to make the sampler tip to reach the drill hole bottom. Similarly, a third drill rod will be added and coupled if the sampler tip still cannot reach the drill hole bottom. This adding, coupling and inserting process will be repeated until the sampler tip reaches the drill hole bottom. This process is the first process of sampler inserting.
Next, once the sampler is placed at the test depth, the impact hammer apparatus will be added to the top of the coupled drill rods and the sampler system. The hammer impact apparatus will be used to make the sampler penetrate into the ground at the drill hole bottom (FIG. 2). The hoist hook will lift the X-clamp upward through the steel chain. The X-clamp will clamp the hammer cap and carry the hammer upward along the guide rod. Once the X-clamp impacts the upper anvil, the clamping at the hammer cap will be forced to open and release the hammer automatically. The hammer will drop freely along the guide rod. The flat bottom surface of the hammer will hit the lower anvil at its flat top surface. The lower anvil bottom is coupled to the drill rods. The induced shock force in the drill rods will make the sampler penetrate into the ground below the drill hole bottom. Once the hammer becomes stable on the lower anvil, the primary operator will drop the hoist hook to make the X-clamp drop onto the hammer cap along the guide rod. Then the operator will tighten the steel chain to make the X-clamp couple the hammer cap again. The operator will then lift the hammer quickly. Again, the hammer will drop freely once the X-clamp impacts the upper anvil. The hammer will hit the lower anvil to make the sampler to penetrate the soil again. The above operation process will be repeated several times until a test criterion is satisfied. This process is the second process of hammer impact and sampler penetrating.
Third, once the penetrating stage is completed, the operators will remove the hammer impact apparatus from the drill rods. The operators will then retrieve the drill rods from the drill hole one by one (FIG. 1). The drill rods and the sampler will be lifted up. The top drill rod will then be decoupled from the remaining drill rods in the drill hole, and it will be placed on the ground nearby. Then the remaining drill rods will be removed from the drill hole. The second top drill rod will be decoupled and placed on the ground nearby. This lifting, decoupling and placing process will be repeated until the first drill rod with the sampler is retrieved from the drill hole. This process is the third process of sampler retrieving. Further drilling work will be then carried out until the bottom end of the drill hole reaches the subsequent test depth. Then the subsequent test will be conducted following the above three processes.
The hammer is made of steel and weighs 63.5 kg. The free drop height is 760 mm. The blow counts of the hammer falling on the anvil are recorded for each of 75 mm penetration between 0 and 450 mm penetrations. The first 150 mm penetration is regarded as a seating drive. The number of blows necessary to drive the sampler to penetrate 300 mm into the ground is known as the penetration resistance or N-value. A specification on how to determine the N-value is normally adopted by authorities for determining the soil shear strength and bearing capacity. A hammer efficiency can be further defined as the percentage ratio of a rod dynamic energy over the total potential energy of the hammer drop height (473 Joule). The rod dynamic energy is calculated from the axial shock force in the drill rod generated by the hammer blowing according to a specific equation such as the equation in ASTM (1995).
The SPT has been widely used and is a tool of choice in Hong Kong housing and infrastructure development as well as landslip preventive measures project. The SPT is included for most ground investigation contracts. The SPT has the following advantages: a) the test apparatus is simple and rugged; b) the test can be carried out in many different types of soils; c) the test has been widely adopted as a routine in-situ testing method throughout the world; and d) tremendous experience and empirical correlations have been obtained for geotechnical design and construction.
The SPT results, and more particularly the N-value and the test depth, however, have been obtained completely from manual measurements. Usually, two contractors conduct the manual measurements. For most tests, there is no full-time independent supervision or inspection. Furthermore, the testing and the drilling are destructive, non-repeatable and time consuming. More importantly, the test is often carried out in colluvium and weathered rock soils in Hong Kong. Gravel, cobbles, and boulders of high strengths and stiffness can appear randomly in the soil. They can substantially alternate the N-values. As a result, the N-values at a construction site can have a large range of variations in Hong Kong.
Therefore, the accuracy and quality of the manual test results have always been the main concern of many geotechnical engineers and contractors in Hong Kong. At present, there is no tool independently to check and verify the accuracy and quality of the manual test results. Therefore, it is believed that automation of the measurement monitoring and recording for SPT can solve the pressing issues and offer additional data for independently checking and verification of the manual test results.